A variety of industrial as well as non-industrial applications use fuel burning boilers which typically operate to convert chemical energy into thermal energy by burning one of various types of fuels, such as coal, gas, oil, waste material, etc. An exemplary use of fuel burning boilers may be in thermal power generators, wherein fuel burning furnaces generate steam from water traveling through a number of pipes and tubes within a boiler, and the generated steam may be then used to operate one or more steam turbines to generate electricity. The electrical energy load (or power output) of a thermal power generator may be a function of the amount of heat generated in a boiler, wherein the amount of heat may be directly determined by the amount of fuel consumed (e.g., burned) per hour, for example.
In many cases, power generating systems include a boiler which has a furnace that burns or otherwise uses fuel to generate heat which, in turn, is transferred to water flowing through pipes or tubes within various sections of the boiler. A typical steam generating system includes a boiler having a superheater section (having one or more sub-sections) in which steam is produced and is then provided to and used within a first, typically high pressure, steam turbine. While the efficiency of a thermal-based power generator is heavily dependent upon the heat transfer efficiency of the particular furnace/boiler combination used to burn the fuel and transfer the heat to the steam flowing within the superheater section or any additional section(s) of the boiler, this efficiency is also dependent on the control technique used to control the temperature of the steam in the superheater section or any additional section(s) of the boiler. To increase the efficiency of the system, the steam exiting the first steam turbine may be reheated in a reheater section of the boiler, which may include one or more subsections, and the reheated steam may be then provided to a second, typically lower pressure steam turbine. However, both the furnace/boiler section of the power system as well as the turbine section of the power system must be controlled in a coordinated manner to produce a desired amount of power.
Moreover, the steam turbines of a power plant are typically run at different operating levels at different times to produce different amounts of electricity or power based on variable energy or load demands provided to the power plant. For example, in many cases, a power plant may be tied into an electrical power transmission and distribution network, sometimes called a power grid, and provides a designated amount of power to the power grid. In this case, a power grid manager or control (dispatch) authority typically manages the power grid to keep the voltage levels on the power grid at constant or near-constant levels (that may be within rated levels) and to provide a consistent supply of power based on the current demand for electricity (power) placed on the power grid by power consumers. Of course, the grid manager typically plans for heavier use and thus greater power requirements during certain times of the days than others, and during certain days of the week and year than others, and may run one or more optimization routines to determine the optimal amount and type of power that needs to be generated at any particular time by the various power plants connected to the grid to meet the current or expected overall power demands on the power grid.
As part of this process, the grid manager typically sends power or load demand requirements (also referred to as load demand set-points or electrical energy load set-points) to each of the power plants supplying power to the power grid, wherein electrical energy load set-points specify the amount of power that each particular power plant may be tasked to provide onto the power grid at any particular time. Of course, to effect proper control of the power grid, the grid manager may send new electrical energy load set-points for the different power plants connected to the power grid at any time, to account for expected and/or unexpected changes in power being supplied to, or consumed from, the power grid. For example, the grid manager may change the electrical energy load set-point for a particular power plant in response to expected or unexpected changes in the demand (which may be typically higher during normal business hours and on weekdays, than at night and on weekends). Likewise, the grid manager may change the electrical energy load set-point for a particular power plant in response to an unexpected or expected reduction in the supply of power on the grid, such as that caused by one or more power units at a particular power plant failing unexpectedly or being brought off-line for normal or scheduled maintenance.
The steam turbine power generation process can be thought of as having two main input process variables—fuel (energy) and turbine throttle valve—and two main output process variables—electrical energy load (megawatt or MW) and turbine steam inlet pressure. For the purpose of achieving high efficiency, many power plants operate in a sliding pressure mode. That is, turbine steam inlet pressure and electrical energy load have a direct, one-to-one relationship at a given operating point (e.g., the rated condition), such that controlling turbine steam inlet pressure is considered equivalent to controlling the electrical energy load. Typically, the relationship can be represented by a curve, where turbine steam inlet pressure is held constant when the electrical energy load is below 40%, and gradually increases as the electrical energy load increases above 40%. In sliding pressure mode, the turbine throttle valve at the inlet to the steam turbine is kept wide open (e.g., 100% open), while the boiler master (fuel) is utilized to control the inlet pressure (also referred to as turbine throttle pressure or turbine steam inlet pressure) to the desired electrical energy load set-point. The power plant controls the turbine steam inlet pressure as the primary output variable rather than electrical energy load, because although the power plant wants to meet the electrical energy load set-point as quickly and efficiently as possible, fast and/or arbitrary movements in the electrical energy load causes the steam pressure variable to swing wildly and uncontrollably due to the one-to-one relationship, thereby creating a safety issue. Controlling turbine steam inlet pressure presents a more reliable and stable manner of controlling the electrical energy load, which is considered more important than speed even though turbine steam inlet pressure is considered a second-best output control variable objective to electrical energy load.
In actual operation, the dispatching center sends the electrical energy load demand signal (e.g., a MW target set-point) to the power plant either by manually calling in or by connecting the demand signal through an Automatic Generation Control (AGC) mechanism. This electrical energy load set-point is converted to a turbine steam inlet pressure set-point in the distributed control system, and the distributed control system controls the pressure in the turbine steam inlet to this set-point. If the electrical energy load (MW) and turbine steam inlet pressure relationship is perfectly lined up, the actual electrical energy load will be controlled to its target.
However, the actual process does not always operate at the rated condition or any other fixed condition. For example, steam temperature and turbine exhaust pressure can deviate significantly from manufacturer design (i.e., the rated condition). Therefore, to maintain an accurate electrical energy load and turbine steam inlet pressure relationship, turbine manufacturers usually supply correction formulas/curves which can be used to modify the turbine steam inlet pressure set-point to achieve the electrical energy load set-point. These formulas are usually characterized by linear and polynomial equations, and are mostly experimentally determined. However, these correction formulas/curves are obtained based on a fixed set of data at the time of manufacture and/or installation. Over time, the unit process characteristics may change slightly, and the electrical energy load and turbine steam inlet pressure relationship needs to be re-calibrated from time-to-time, perhaps at various operating points. A multivariate linear regression model of the relationship between the turbine steam inlet pressure and the electrical energy load has been used in real-time with the steam turbine power generation process to better track this relationship and how the relationship changes over time. It works well in most conditions, but in certain conditions the actual electrical energy load is off from the electrical energy load set-point by as much as 2 MW. This difference results from an inaccurate electrical energy load and turbine steam inlet pressure relationship obtained by the linear multivariate regression method.